In order to achieve improved efficiency in incandescent lamp filaments, filament design has progressed toward more compact coil configurations, starting with filament coiling as taught by Langmuir in U.S. Pat. No. 1,180,159. Coiled coil filaments as taught by Benbow in U.S. Pat. No. 1,247,068 also exhibit improved efficiency. Efforts to achieve compact coiled coil designs for the finer more resistive wires (having a diameter of 4.5 mils or less) had been limited by the need to impart rigidity to the filament by using the smallest possible mandrels for the primary and secondary coiling.
The use of coiling in filaments shortens the filament and increases its diameter. A wire of diameter D is coiled about a mandrel having a diameter M. The resulting coil is said to be compressed and is obviously shorter than the original wire and has a diameter of D.sub.1 =2d+M. In order to impart rigidity to the coil, it has been common coil practice to keep the diameter of the coil small. This is accomplished by keeping the mandrel ratio (M/d) as small as possible. However, a wire of diameter d cannot without special preparations be coiled about a mandrel having a diameter M which is less than d.
In U.S. Pat. No. 4,208,609 to Berlec, there is disclosed a low-power incandescent lamp having a reduction in filament squirm due to the fact that the filament has higher pitch ratios, lower mandrel ratios and is mounted under tension between the lead wires. The teaching in the Berlec Patent apply specifically to lamps of low power and having filament diameters of 1 mil or less. Triple coils have also been proposed to obtain even greater efficiency by means of an even more compact coil. However, triple coils as taught by Graves et al. in U.S. Pat. Nos. 4,316,116 and 4,449,401 have been unable to achieve sufficient rigidity to support finer, more resistant filament wires unless the mandrel ratios in either or both its secondary and tertiary coiling are less than 1. Such coils are difficult to wind and, as acknowledged by the Graves U.S. Pat. No. 4,499,401 following the description in the Graves U.S. Pat. No. 4,316,116 has lead to triple coil filaments having a flimsy structure when arranged within the incandescent lamp which causes sagging and causes the filament to ultimately experience a burn out.
Incandescent lamps of the tungsten halogen variety generally have superior performance characteristics over traditional Edison-type incandescent lamps because the former can be back-filled to very high gas pressures. This higher fill pressure retards the evaporation rate of the operating filament thus prolonging its life. However, providing lamps with long life and compact filaments is still a major problem with tungsten halogen lamps, especially with fine wire incandescent lamps. One way of extending the operating life of a filament and the incandescent lamp is to reduce the voltage across the filament by placing recitfying means, such as a diode, in series with the filament. The filament is operated at a substantially lower temperature and also at a lower efficiency in terms of lumens per watt. As an example, U.S. Pat. No. 3,869,631 by Anderson et al. teaches that an incandescent lamp having a diode in series with the tungsten filament will provide improved luminous efficacy without reducing lamp life provided the filament weight is increased approximately 50% over that ordinarily used with the particular lamp.
It is believed, therefore, that an incandescent lamp filament which achieves a high degree of compactness for the finer more resistive wires while retaining or increasing structural rigidity in an incandescent lamp would constitute an advancement in the art. In addition, a filament design which would promote smaller capsule design for high pressure halogen lamps and would provide for a simpler and less expensive mounting arrangements for the filament would constitute an even further advancement in the art.